Tubing used in a motor vehicle is frequently exposed to alcohol. The motor-vehicle fuel itself frequently contains alcohol-type additives serving as dryers or can even be in large part formed by alcohol itself serving as fuel, the brake fluid includes alcohol to prevent any condensate in the brake lines from freezing up, and the fluid used in the windshield washer includes alcohol both to prevent freezing and act as a solvent. The instant invention is aimed at tubing serving all these and other purposes, both as original equipment or as replacement part.
The standard such tubing as described in German patent 3,821,723 filed Jun. 28, 1988 by E. Brunnhofer comprises three coextruded layers. The outer layer is relatively thick and is formed of polyamide, typically Nylon 12 which is a type of nylon made by the polymerization of lauric lactam (dodecanoic lactam) or cyclodedecalactam, with 11 methylene units between the linking --NH--CO-- groups in the polymer chain. The mechanical properties of Nylon 12 are intermediate between those of conventional nylons and polyethylene, and it is the lowest in water absorption (1.5%) and specific gravity (1.01) of all the nylons (Whittington's Dictionary of Plastics by L. Whittington; Technomic 1968). The inner layer is much thinner and is formed of Nylon 6, Nylon 6.6, Nylon 11 which is a type of nylon derived from 11-aminoundecanoic acid, used for fibers and moldings (Whittington's Dictionary of Plastics; op. Cit.), or Nylon 12. The intermediate layer is made of a polyolefin and acts as a block to the perfusion of alcohol to the outer layer, so that the thick outer layer will impart dimensional stability to the hose.
Particular problems arise, however, when surface-active agents or oxygen-releasing substances pass through such a tubing. In fact cracks and oxidation of the tubing is possible. Furthermore the plastic of the tubing contains substances, monomers or oligomers, which are soluble in the fuel or other liquid in the tubing. Not only does this have a bad effect on the service life of the tubing, but when the vehicle equipped with such tubing is first filled with, for instance, gasoline and then allowed to stand a long time, for instance, for shipping overseas, the composition of the fuel or other liquid in the tubing changes as components of the tubing diffuse into the liquid in the tubing. Alcohol and aromatics in the fluid diffuse at different rates through the layered tubing wall from the aliphatic components. The resultant change in the composition of the liquid in the tubing changes the solubility thresholds of them so as, for example, to be able to crystalize monomers and oligomers of the Nylon 11 and Nylon 12 into the liquid. The presence of copper ions, which can be picked up from the fuel pump, accelerates this crystallization. The crystallized precipitate can block filters and collect to limit travel of the fuel-pump or carburetor float, and can plug jets or injectors as well as build up on critical control surfaces of the fuel pump.